The patch is about 5 megs in size. We do not have a download mirror, as it appears to only update the launcher.
While there is some speculation that the launcher is being prepared to get ready for the 3.1 PTR, all that is speculation. We do however, expect the 3.1 PTR to drop sometime soon. In the mean time you can check out our list of known patch 3.1 changes.
If there is any information from Blizzard on the patch, we'll update this post.